Langmuir probe interpretation for plasmas with secondary electrons from the wall

Abstract

A method is presented for analyzing the electron current to a cylindrical Langmuir probe in a low pressure, hot-filament discharge plasma containing secondary electrons from the wall in addition to colder bulk plasma electrons. Orbit-motion-limited probe theory is applied to each of the electron components, taking into consideration that the secondary electron current is in the saturation region for probe potentials more positive than the wall potential. The method resolves the probe current into ion, secondary electron, and bulk electron components and finds parameters for each. The fitted model curve follows the probe data with less than 5% relative error from below the floating potential to the saturation region. The analysis shows that the probe current of the bulk electrons alone is indistinguishable from zero for probe potentials more negative than the wall potential, indicating that there are indeed no bulk electrons with energies exceeding the ambipolar potential.